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This problem is addressed in ArcGIS 8.1 Service Pack 1. See the Related Information section to download this pack.
If a shapefile is used as a mask for an analysis operation, an error, "The cell size value is either invalid or not defined", is returned when using the Raster Calculator.
This is a known issue with ArcGIS 8.1.
This is fixed in ArcGIS 8.1 Service Pack 1.
Alternatively, specify a cell size in the Cell Size tab of the Options dialog before performing an operation in the Raster Calculator using a mask.